Ink formulations for improving printhead lifetime

ABSTRACT

An inkjet ink includes: (i) a disazo dye of formula (I): 
     
       
         
         
             
             
         
       
     
     (ii) a glycol compound selected from the group consisting of: triethylene glycol, tetraethylene glycol and pentaethylene glycol; and (iv) water.

This application is a continuation of U.S. application Ser. No.14/640,244, entitled INK FORMULATIONS FOR IMPROVING PRINTHEAD LIFETIME,filed Mar. 6, 2015, which claims priority to and the benefit of U.S.Provisional Patent Application No. 61/971,985, entitled INK FORMULATIONSFOR IMPROVING PRINTHEAD LIFETIME, filed Mar. 28, 2014, the disclosuresof each of which are incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

This invention relates to an inkjet ink. It has been developed primarilyfor improving the lifetime of printheads, and particularly theApplicant's Memjet® printheads.

BACKGROUND OF THE INVENTION

The present Applicant has developed a plethora of high-speed inkjetprinters employing stationary Memjet® printheads which extend across amedia width. By contrast, virtually all other types of inkjet printerutilize a scanning printhead which traverses across the media width.

High-speed pagewidth printing necessarily places additional demands onthe design of the printhead compared to traditional types of inkjetprinthead. The nozzle devices must have a self-cooling design, high inkrefill rates and high thermal efficiency. To this end, the Applicant hasdeveloped a range of thermal bubble-forming printheads, including thosewith suspended resistive heater elements (as described in, for example,U.S. Pat. No. 6,755,509; U.S. Pat. No. 7,246,886; U.S. Pat. No.7,401,910; and U.S. Pat. No. 7,658,977, the contents of which areincorporated herein by reference) and those with embedded (“bonded”)resistive heater elements (as described in, for example, U.S. Pat. No.7,377,623; U.S. Pat. No. 7,431,431; US 2006/250453; and U.S. Pat. No.7,491,911, the contents of which are incorporated herein by reference).

Nozzle devices having suspended heater elements offer the advantages ofefficient heat transfer from the heater element to the ink andself-cooling characteristics. However, they suffer from the disadvantageof relatively short printhead lifetimes, because suspended heaterelements are typically less robust than their bonded counterparts.

One approach to improving printhead lifetime is to coat the heaterelements with a layer of protective coating. For example, U.S. Pat. No.6,719,406 (assigned to the present Applicant) describes suspended heaterelements having a conformal protective coating, which improves therobustness of the heater element and improves printhead lifetime.However, protective coatings are undesirable for a number ofreasons—they reduce the efficiency of heat transfer from the resistiveheater elements to the surrounding ink; they consequently affect theself-cooling characteristics; and they introduce additional MEMSfabrication challenges.

Therefore, it is generally preferable to employ uncoated (“naked”)heater elements in Memjet® printheads with a consequential reduction inprinthead lifetime. To some extent, the choice of heater material canmitigate the effects of using uncoated heater elements. For example,U.S. Pat. No. 7,431,431 describes the use of a self-passivating titaniumaluminium nitride heater element, which has improved lifetime comparedto more conventional materials, such as titanium nitride. Nevertheless,there is still a need to improve the lifetimes of Memjet® printheads,and particularly those employing uncoated heater elements.

It has been found that certain inks are particularly aggressive towardsheater elements. For example, many dye-based inks have been found tocorrode heater elements resulting in shortened printhead lifetimes. In amulti-color printhead (e.g. CMYK), the printhead lifetime is, to a largeextent, limited by the lifetime of the color channel having the shortestlifetime. If, for example, a black dye-based ink is found to beparticularly corrosive towards heater elements, then the lifetime of theprinthead will be determined by the lifetime of the black channel, evenif all other color channels still perform well when the black colorchannel fails.

In the present context, “failure” of a nozzle device means any change indrop ejection characteristics which results in unacceptable printquality. For example, failure may be invoked by a reduction in dropvelocity, poor drop directionality or non-ejection of ink. Moreover, thecriteria for failure may be different for different colors. For example,a reduction in print quality in a yellow channel may be more tolerablethan a corresponding reduction in print quality in a black channel,because black ink is more visible to the human eye (i.e. black ink has ahigher luminance on white paper). This, in combination with theaggressive nature of many black dyes, means that the black channel in aMemjet® printhead is typically the limiting color channel in terms ofprinthead lifetime.

It would be desirable to improve the lifetime of printheads employingresistive heater elements. It would be further desirable to improve thelifetime of such printheads without modifying the design of theprinthead.

By way of background, US2007/0050926 and US2012/0090503 describes inkjetinks comprising disazo dyes having a central arylene motif of theformula:

These disazo dyes described in the prior art are formulated in inkvehicles comprising ethylene glycol and diethylene glycol co-solvents.

U.S. Provisional Application No. 61/844,276 filed on 9 Jul. 2013, thecontents of which are incorporated herein by reference, describes theuse of ethoxylated butyne-1,2-diols as an anti-corrosion additive forimproving printhead lifetime.

SUMMARY OF THE INVENTION

In a first aspect, the present invention provides an inkjet inkcomprising: (i) a disazo dye of formula (I):

wherein:A is a C₆₋₁₄ aryl group having 0, 1, 2, 3, 4 or 5 substituents selectedfrom the group consisting of:—SO₃M, nitro, carboxyl, halogen, hydroxyl, amino, C₁₋₄alkylamino, C₁₋₄alkoxy, C₁₋₄ alkyl, C₁₋₄ haloalkyl, cyano, sulfonamide, carbamoyl, C₁₋₄alkylamido and C₁₋₄alkoxycarbonyl;B is a C₆₋₁₄ aryl group having 0, 1, 2, 3, 4 or 5 substituents selectedfrom the group consisting of:—SO₃M, nitro, carboxyl, halogen, hydroxyl, amino, C₁₋₄alkylamino, C₁₋₄alkoxy, C₁₋₄ alkyl, C₁₋₄ haloalkyl, cyano, sulfonamide, carbamoyl, C₁₋₄alkylamido and C₁₋₄ alkoxycarbonyl;each M is independently selected from the group consisting of: hydrogen,lithium, sodium, potassium, ammonium and quaternary ammonium; andthe dye of formula (I) comprises at least 3 groups of formula —SO₃M;(ii) sulfolane;(iii) a glycol compound selected from the group consisting of: ethyleneglycol, diethylene glycol, triethylene glycol, tetraethylene glycol andpentaethylene glycol; and(iv) water.

Preferred embodiments of the disazo dye colorant are described in moredetail below.

The disazo dye may be present in an amount ranging from 0.01 to 25 wt.%. Preferably, the disazo dye is present in an amount ranging from 1 to10 wt. %, or preferably 2 to 8 wt. %.

Preferably, the disazo dye is a black dye.

Sulfolane (2,3,4,5-tetrahydrothiophene-1,1-dioxide) is a co-solventwhich may be present in an amount ranging from 5 to 40 wt. %, orpreferably from 10 to 30 wt. %.

The glycol compound is a co-solvent which may be present in an amountranging from 1 to 20 wt. %, preferably 1 to 10 wt. %, or preferably 2 to6 wt. %.

Preferably, the ink comprises a higher order glycol, such as triethyleneglycol or tetraethylene glycol, more preferably triethylene glycol. TheApplicant's experiments have shown that higher order glycols, incombination with sulfolane, are consistently efficacious in improvingprinthead lifetime and generally outperform inks comprising only lowerorder glycols. Moreover, higher order glycols impart other usefulproperties to the ink, such as humectancy and anti-curl properties. Theanti-curl properties of higher order glycols are described in, forexample, U.S. Pat. No. 5,356,464. Mixtures of glycol compounds are, ofcourse, contemplated within the scope of the present invention.

Preferably, an amount of sulfolane exceeds an amount of the glycolcompound. Preferably, a ratio of sulfolane to the glycol compound is inthe range of 10:1 to 2:1, or preferably 8:1 to 4:1

Preferably, the ink vehicle comprises 0.1 to 30 wt. % of one or moreother co-solvents—that is, co-solvents other than the sulfolane andglycol co-solvents described above. The range of other co-solvents isnot particularly limited, and some suitable other co-solvents for use inthe present invention are described in more detail below.

Preferably, the ink vehicle comprises 0.05 to 2 wt % of at least onesurfactant. Preferably, the ink vehicle comprises 0.1 to 1 wt % ofsurfactant. The range of surfactants is not particularly limited andsome suitable surfactants for use in the present invention are describedin more detail below. For example, the surfactant may be an anionic,cationic, nonionic or zwitterionic surfactant. Nonionic surfactantshaving an acetylenic moiety (e.g. the Surfynol® range of surfactants,available from Air Products and Chemicals, Inc.) are generallypreferred.

The ink may further comprise a corrosion inhibitor. Corrosion inhibitorsmay be employed to further improve printhead lifetimes. Preferably, thecorrosion inhibitor comprises an acetylenic compound, such as thosedescribed in U.S. Provisional Application No. 61/844,276 filed on 9 Jul.2013. Preferably, the corrosion inhibitor comprises an alkoxylatedacetylenic diol.

As described in U.S. Provisional Application No. 61/844,276, suitablealkoxylated acetylenic diols include compounds of formula (A):

R¹—C≡C—(CH₂)_(m)CH(R²)—(OCH₂R³)n-OH  (A)

wherein:

R¹ is selected from the group consisting of: H, C₁₋₃ alkyl; and—(CH₂)_(p)CH(R⁴)(R⁵);

R² is selected from the group consisting of: H and C₁₋₃ alkyl;

R³ is selected from the group consisting of: —CH₂— and —CH(CH₃)—;

R⁴ is selected from the group consisting of: H and C₁₋₃ alkyl;

R⁵ is selected from the group consisting of: —OH and —(OCH₂R³)_(q)—OH;

m is 0, 1, 2 or 3;

n is 1 to 50;

p is 0, 1, 2 or 3; and

q is 1 to 50.

The alkoxylated acetylenic diol may be an ethoxylated butynediol, suchas a compound of formula (B):

R⁶—C≡C—(CH₂)(OCH₂CH₂OH)  (B)

wherein:

R⁶ is selected from the group consisting of: —CH₂OH and —CH₂(OCH₂CH₂OH).

Typically, the corrosion inhibitor, when present, comprises1,4-bis(2-hydroxyethoxy)-2-butyne).

In a second aspect, there is provided a method of improving a lifetimeof an inkjet printhead, the method comprising the steps of:

supplying an ink as described above to nozzle chambers of the printhead,each nozzle chamber having an associated actuator in contact with theink; and

actuating one or more of the actuators to eject ink from the printhead.

The method according to second aspect significantly improves printheadlifetimes compared to methods employing inks comprising disazo dyes andco-solvents not in accordance with the ink described above.

Preferably, each actuator comprises a resistive heater element, whichsuperheats ink so as to form a bubble and eject ink from thecorresponding nozzle chamber via a nozzle opening.

Preferably, the heater element is uncoated so that the ink is in directcontact with the heater element material. The heater element may besuspended in the nozzle chamber or bonded to a wall (e.g. floor) of thenozzle chamber.

Preferably, the heater element is comprised of a metal or a conductiveceramic material, such as a metal nitride. As used herein, the term“metal” includes metal alloys containing a plurality of differentmetals. Preferably, the heater element is comprised of a materialselected from the group consisting of: a titanium alloy (e.g.titanium-aluminium alloy); titanium nitride; and a nitride of a titaniumalloy (e.g. titanium aluminium nitride).

In a third aspect, there is provided an inkjet printer comprising:

-   -   an inkjet printhead having a plurality of nozzle chambers, each        nozzle chamber having an associated actuator for contacting ink;        and    -   an ink reservoir in fluid communication with the nozzle        chambers, the ink reservoir containing an ink as described        above.

In a fourth aspect, there is provided a use of an ink as describedherein, or use of a disazo dye of formula (I), for improving a lifetimeof an inkjet printhead.

In a fifth aspect, there is provided a method of improving a lifetime ofan inkjet printhead, the method comprising the steps of:

supplying an ink to nozzle chambers of the printhead, each nozzlechamber having an uncoated heater element in contact with the ink; and

actuating one or more of the heater elements to eject ink from theprinthead, wherein the ink comprises:

(i) a black disazo dye;(ii) sulfolane;(iii) a higher order glycol compound selected from the group consistingof: triethylene glycol, tetraethylene glycol and pentaethylene glycol;and(iv) water.

In accordance with the fifth aspect, the black disazo dye is notparticularly limited and may be, for example, a dye according to formula(I) or another disazo dye, such as Food Black 2.

Preferred embodiments of the second, third, fourth and fifth aspectswill be readily apparent from the foregoing. Preferred embodiments,where described, are not intended to be limited to any particular aspectof the present invention.

As used herein, the term “aryl” refers to an aromatic group, such asphenyl, naphthyl or anthracenyl. Unless otherwise specified, the term“aryl” generally refers to C₆₋₁₄ aryl groups.

As used herein, the term “alkyl” refers to alkyl groups in both straightand branched forms. Unless otherwise specified, the term “alkyl”generally refers to C₁₋₄ alkyl groups.

As used herein, the term “halogen” or “halo” refers to any of fluorine,chlorine, bromine and iodine. Usually, the term “halogen” to chlorine orfluorine substituents.

As used herein, the term “haloalkyl” refers to alkyl groups having oneor more halogen substituents e.g. trifluoromethyl.

As used herein, the term “carboxyl” refers to carboxylic acid groups inprotonated or salt form. Generally, carboxyl groups are —CO₂M groups,wherein M is as defined hereinabove.

As used herein, the term “alkylamino” refers to both mono-alkylamino anddi-alkylamino groups.

As used herein, the term “sulphonamide” refers to aminosulfonyl andalkylaminosulfonyl groups.

As used herein, the term “carbamoyl” refers to aminocarbonyl andalkylaminocarbonyl groups.

As used herein, the term “ammonium” generally refers to NH₄ ⁺ cations.The term “quaternary ammonium” generally refers to cations of formulaN(R^(a))₄ ⁺, wherein each R^(a) is independently selected from the groupconsisting of C₁₋₆ alkyl, aryl and arylalkyl. A typical example of aquaternary ammonium ion is tetramethylammonium.

By convention, sulfonate groups are represented as —SO₃M in formula (I)above. However, it will of course be appreciated that such groupsusually exist in solution as ion pairs i.e. —SO₃ ⁻M⁺. Thus, whenreference is made to M being an alkali metal, it is implicit thatreference is being made to the corresponding alkali metal ion M⁺, suchas Li⁺, Na⁺ or K⁺.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the present invention will now be described byway of example only with reference to the accompanying drawings, inwhich:—

FIG. 1 is a perspective view of part of a thermal inkjet printhead;

FIG. 2 is a side view of one of the nozzle assemblies shown in FIG. 1;

FIG. 3 is a perspective of the nozzle assembly shown in FIG. 2; and

FIG. 4 is perspective view of a thermal inkjet print engine.

DETAILED DESCRIPTION OF THE INVENTION

The present inventors have sought a solution to the problem of improvingprinthead lifetime by investigating dye and co-solvent combinations. Asforeshadowed above, an ink formulation is an attractive solution to theproblem of printhead lifetime, because it does not require anymodifications to the design of the printhead. However, hitherto, thecorrosiveness of inkjet dyes was understand to be related to thepresence of alkali metal ions, such as sodium and potassium, or thepresence of reactive functional groups on the dye molecule (e.g. C.I.Reactive Black 5). Therefore, attempts to improve printhead lifetimehave usually focused on ink additives for suppressing known corrosionmechanisms.

Sulfonated disazo dyes are an attractive class of dyes for use inMemjet® printheads, because they exhibit excellent rehydrationcharacteristics. A non-functioning printhead nozzle which has becomeblocked with dehydrated ink will become functioning again during routinemaintenance operations if the dye has suitable rehydrationcharacteristics. By contrast, nozzles blocked with inks having poorrehydration characteristics (e.g. inks containing trisazo dyes) are moreproblematic and cannot be readily recovered during routine maintenanceoperations.

Sulfonated disazo dyes are a well known class of dye and include, forexample, Food Black 2:

However, whilst Food Black 2 has excellent rehydration characteristicsand acceptable black optical density for most applications, conventionalinks formulated with Food Black 2 (sodium salt) generally exhibit verypoor printhead lifetimes of the order of <20 million ejections inMemjet® printheads. Substituting the sodium salt of Food Black 2 withother counterions, such as tetramethylammonium produces a markedimprovement in printhead lifetime. Therefore, it was initiallyconsidered that the alkali metal ion content of ink is primarilyresponsible for heater corrosion in inkjet nozzle devices, consistentwith other hypotheses reported in the literature (see, for example, U.S.Pat. No. 6,252,617 which advocates an alkali metal ion content of lessthan 5×10⁻³ mol/l for improved printhead lifetime).

Although experiments by the Applicant showed that printhead lifetimesassociated with Food Black 2 could be improved by substituting sodiumsions with tetramethylammonium ions, the relatively poor stability ofFood Black 2 towards light led the present inventors to consider similardisazo dyes having reportedly improved lightfastness. For example,US2007/0050926 and US2012/0090503 describe disazo dyes having adifferent central arylene motif than Food Black 2, and which haveexcellent lightfastness.

Surprisingly, it was found that inks comprising disazo dyes of formula(I) generally exhibited superior printhead lifetimes compared to inkscomprising Food Black 2, even though both dyes were in their sodium saltform, and both dyes are similar highly sulfonated disazo compounds.Therefore, in contrast with reports in the literature (e.g. U.S. Pat.No. 6,252,617), the presence or otherwise of sodium ions may not be theprimary modulator of heater corrosion mechanisms and, in fact, thenature of the organic dye structure appears to be a more significantfactor, at least when used in the Applicant's Memjet® printheads.

What is more surprising is that the presence of certain co-solvents inthe ink vehicle also has a significant effect on printhead lifetimes forthose inks containing disazo dyes. In particular, the presence ofsulfolane and a glycol compound (e.g. triethylene glycol) in combinationprovided an apparently synergistic effect in terms of improved printheadlifetime. Inks formulated with sulfolane only or ethylene glycol onlydid not show the same improvement as the combination. The surprisingeffectiveness, in terms of improved printhead lifetime, of certainco-solvent combinations with certain disazo dyes was not at allpredictable from the prior art.

Thus, the present invention provides inks exhibiting improved printheadlifetimes, as well as excellent lightfastness and rehydrationcharacteristics. Moreover, the inks according to the present inventiondo not require inconvenient and potentially expensive substitution ofalkali metal ions in order to achieve improved printhead lifetimes.

Colorant

As described above, the colorant in the inks according to the presentinvention is a disazo dye of formula (I).

Preferably, each of A and B has at least one substituent of formula—SO₃M, such that the dye of formula (I) comprises at least 4 groups offormula —SO₃M.

Preferably, A is a phenyl or naphthyl group having 1, 2 or 3substituents selected from the group consisting of: —SO₃M, nitro,carboxyl, halogen, hydroxyl, amino, C₁₋₄ alkoxy and C₁₋₄ alkyl.

Preferably, A is a phenyl group having 1, 2 or 3 substituents selectedfrom the group consisting of: —SO₃M, nitro and carboxyl, and morepreferably the group consisting of: —SO₃M and nitro.

Preferably, A is a phenyl group having 2 substituents selected from thegroup consisting of: —SO₃M and nitro.

Preferably, B is a phenyl or naphthyl group having 1, 2 or 3substituents selected from the group consisting of: —SO₃M, nitro,carboxyl, halogen, hydroxyl, amino, C₁₋₄ alkoxy and C₁₋₄ alkyl.

Preferably, B is a phenyl group having 1, 2 or 3 substituents selectedfrom —SO₃M, nitro and carboxyl, and more preferably the group consistingof: —SO₃M and nitro.

Preferably, B is a phenyl group having 2 substituents selected from thegroup consisting of: —SO₃M and nitro.

Preferably, A and B are different than each other.

Preferably, M is sodium.

Preferably, the disazo dye is of formula (II):

The disazo dyes of formula (I) may be used in inkjet inks eitherindividually or as a combination of two or more thereof. Moreover, theinks according to the present invention may comprise other colorants,such as dyes used for tinting the ink so as to optimize hue and chroma.Suitable tinting dyes will be readily apparent to the person skilled inthe art.

Ink Vehicle

The ink vehicles used in the present invention are typicallyconventional aqueous ink vehicles comprising at least 40 wt % water, atleast 50 wt % water or at least 60 wt % water. Usually, the amount ofwater present in the inkjet ink is in the range of 40 wt % to 90 wt %,or optionally in the range of 50 wt % to 70 wt %.

The inks according to the present invention comprise at least sulfolaneand the glycol compound as co-solvents. The inks according to thepresent invention may further comprise other co-solvents (includinghumectants, penetrants, wetting agents etc.), surfactants, biocides,sequestering agents, pH adjusters, viscosity modifiers, etc.

Co-solvents are typically water-soluble organic solvents. Suitablewater-soluble organic solvents include C₁₋₄ alkyl alcohols, such asethanol, methanol, butanol, propanol, and 2-propanol; glycol ethers,such as ethylene glycol monomethyl ether, ethylene glycol monoethylether, ethylene glycol monobutyl ether, ethylene glycol monomethyl etheracetate, diethylene glycol monomethyl ether, diethylene glycol monoethylether, diethylene glycol mono-n-propyl ether, ethylene glycolmono-isopropyl ether, diethylene glycol mono-isopropyl ether, ethyleneglycol mono-n-butyl ether, diethylene glycol mono-n-butyl ether,triethylene glycol mono-n-butyl ether, ethylene glycol mono-t-butylether, diethylene glycol mono-t-butyl ether, 1-methyl-1-methoxybutanol,propylene glycol monomethyl ether, propylene glycol monoethyl ether,propylene glycol mono-t-butyl ether, propylene glycol mono-n-propylether, propylene glycol mono-isopropyl ether, dipropylene glycolmonomethyl ether, dipropylene glycol monoethyl ether, dipropylene glycolmono-n-propyl ether, dipropylene glycol mono-isopropyl ether, propyleneglycol mono-n-butyl ether, and dipropylene glycol mono-n-butyl ether;formamide, acetamide, dimethyl sulfoxide, sorbitol, sorbitan, glycerolmonoacetate, glycerol diacetate, glycerol triacetate, and sulfolane; orcombinations thereof.

Other useful water-soluble organic solvents, which may be used asco-solvents, include polar solvents, such as 2-pyrrolidone,N-methylpyrrolidone, ε-caprolactam, dimethyl sulfoxide, morpholine,N-ethylmorpholine, 1,3-dimethyl-2-imidazolidinone and combinationsthereof.

In addition to the glycol compounds, the inkjet ink may contain anotherhigh-boiling water-soluble organic solvent as a co-solvent, which canserve as a wetting agent or humectant for imparting water retentivityand wetting properties to the ink composition. Examples of high-boilingwater-soluble organic solvents are 2-butene-1,4-diol,2-ethyl-1,3-hexanediol, 2-methyl-2,4-pentanediol, tripropylene glycolmonomethyl ether, dipropylene glycol monoethyl glycol, dipropyleneglycol monoethyl ether, dipropylene glycol monomethyl ether, dipropyleneglycol, triethylene glycol monomethyl ether, diethylene glycol monobutylether, diethylene glycol monoethyl ether, diethylene glycol monomethylether, tripropylene glycol, polyethylene glycols having molecularweights of 2000 or lower, 1,3-propylene glycol, isopropylene glycol,isobutylene glycol, 1,4-butanediol, 1,3-butanediol, 1,5-pentanediol,1,6-hexanediol, glycerol, trimethylolpropane, erythritol,pentaerythritol and combinations thereof.

Other suitable wetting agents or humectants include saccharides(including monosaccharides, oligosaccharides and polysaccharides) andderivatives thereof (e.g. maltitol, sorbitol, xylitol, hyaluronic salts,aldonic acids, uronic acids etc.)

The inkjet ink may also contain a penetrant, as one of the co-solvents,for accelerating penetration of the aqueous ink into the recordingmedium. Suitable penetrants include polyhydric alcohol alkyl ethers(glycol ethers) and/or 1,2-alkyldiols. Examples of suitable polyhydricalcohol alkyl ethers are ethylene glycol monomethyl ether, ethyleneglycol monoethyl ether, ethylene glycol monobutyl ether, ethylene glycolmonomethyl ether acetate, diethylene glycol monomethyl ether, diethyleneglycol monoethyl ether, ethylene glycol mono-n-propyl ether, ethyleneglycol mono-isopropyl ether, diethylene glycol mono-isopropyl ether,ethylene glycol mono-n-butyl ether, diethylene glycol mono-n-butylether, triethylene glycol mono-n-butyl ether, ethylene glycolmono-t-butyl ether, diethylene glycol mono-t-butyl ether,1-methyl-1-methoxybutanol, propylene glycol monomethyl ether, propyleneglycol monoethyl ether, propylene glycol mono-t-butyl ether, propyleneglycol mono-n-propyl ether, propylene glycol mono-isopropyl ether,dipropylene glycol monomethyl ether, dipropylene glycol monoethyl ether,dipropylene glycol mono-n-propyl ether, dipropylene glycolmono-isopropyl ether, propylene glycol mono-n-butyl ether, anddipropylene glycol mono-n-butyl ether. Examples of suitable1,2-alkyldiols are 1,2-pentanediol and 1,2-hexanediol. The penetrant mayalso be selected from straight-chain hydrocarbon diols, such as1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol,1,7-heptanediol, and 1,8-octanediol. Glycerol may also be used as apenetrant.

Typically, the total amount of co-solvent (including sulfolane, glycolcompound and other co-solvents) present in the ink is in the range ofabout 10 wt % to 60 wt %, or optionally 15 wt % to 50 wt %.

The inkjet ink may also contain one or more surface active agents(“surfactant”), such as an anionic surface active agent, a zwitterionicsurface active agent, a nonionic surface active agent or mixturesthereof. Useful anionic surface active agents include sulfonic acidtypes, such as alkanesulfonic acid salts, α-olefinsulfonic acid salts,alkylbenzenesulfonic acid salts, alkylnaphthalenesulfonic acids,acylmethyltaurines, and dialkylsulfosuccinic acids; alkylsulfuric estersalts, sulfated oils, sulfated olefins, polyoxyethylene alkyl ethersulfuric ester salts; carboxylic acid types, e.g., fatty acid salts andalkylsarcosine salts; and phosphoric acid ester types, such asalkylphosphoric ester salts, polyoxyethylene alkyl ether phosphoricester salts, and glycerophosphoric ester salts. Specific examples of theanionic surface active agents are sodium dodecylbenzenesulfonate, sodiumlaurate, and a polyoxyethylene alkyl ether sulfate ammonium salt.

Examples of zwitterionic surface active agents includeN,N-dimethyl-N-octyl amine oxide, N,N-dimethyl-N-dodecyl amine oxide,N,N-dimethyl-N-tetradecyl amine oxide, N,N-dimethyl-N-hexadecyl amineoxide, N,N-dimethyl-N-octadecyl amine oxide andN,N-dimethyl-N—(Z-9-octadecenyl)-N-amine oxide.

Examples of nonionic surface active agents include ethylene oxide adducttypes, such as polyoxyethylene alkyl ethers, polyoxyethylene alkylphenylethers, polyoxyethylene alkyl esters, and polyoxyethylene alkylamides;polyol ester types, such as glycerol alkyl esters, sorbitan alkylesters, and sugar alkyl esters; polyether types, such as polyhydricalcohol alkyl ethers; and alkanolamide types, such as alkanolamine fattyacid amides. Specific examples of nonionic surface active agents areethers such as polyoxyethylene nonylphenyl ether, polyoxyethyleneoctylphenyl ether, polyoxyethylene dodecylphenyl ether, polyoxyethylenealkylallyl ether, polyoxyethylene oleyl ether, polyoxyethylene laurylether, and polyoxyalkylene alkyl ethers (e.g. polyoxyethylene alkylethers); and esters, such as polyoxyethylene oleate, polyoxyethyleneoleate ester, polyoxyethylene distearate, sorbitan laurate, sorbitanmonostearate, sorbitan mono-oleate, sorbitan sesquioleate,polyoxyethylene mono-oleate, and polyoxyethylene stearate.

Acetylene glycol surface active agents, such as2,4,7,9-tetramethyl-5-decyne-4,7-diol; ethoxylated2,4,7,9-tetramethyl-5-decyne-4,7-diol; 3,6-dimethyl-4-octyne-3,6-diol or3,5-dimethyl-1-hexyn-3-ol, may also be used. Specific examples ofnonionic surfactants, which may be used in the present invention, areSurfynol® 465 and Surfynol® 440 (available from Air Products andChemicals, Inc).

The surfactant(s) are typically present in the aqueous inkjet ink in anamount ranging from 0.05 wt. % to 2 wt % or 0.1 to 1 wt. %.

The aqueous inkjet ink may also include a pH adjuster or buffer, such assodium hydroxide, potassium hydroxide, lithium hydroxide, sodiumcarbonate, sodium hydrogencarbonate, potassium carbonate, potassiumhydrogencarbonate, lithium carbonate, sodium phosphate, potassiumphosphate, lithium phosphate, potassium dihydrogenphosphate, dipotassiumhydrogenphosphate, sodium oxalate, potassium oxalate, lithium oxalate,sodium borate, sodium tetraborate, potassium hydrogenphthalate, andpotassium hydrogentartrate; ammonia; and amines, such as methylamine,ethylamine, diethylamine, trimethylamine, triethylamine,tris(hydroxymethyl)aminomethane hydrochloride, triethanolamine,diethanolamine, diethylethanolamine, triisopropanolamine,butyldiethanolamine, morpholine, propanolamine,4-morpholineethanesulfonic acid and 4-morpholinepropanesulfonic acid(“MOPS”). The amount of pH adjuster, when present, is typically in therange of from 0.01 to 2 wt. % or 0.05 to 1 wt. %.

The aqueous inkjet ink may also include a biocide, such as benzoic acid,dichlorophene, hexachlorophene, sorbic acid, hydroxybenzoic esters,sodium dehydroacetate, 1,2-benthiazolin-3-one (“Proxel® GXL”, availablefrom Arch Chemicals, Inc.), 3,4-isothiazolin-3-one or4,4-dimethyloxazolidine. The amount of biocide, when present, istypically in the range of from 0.01 to 2 wt. % or 0.05 to 1 wt. %.

The aqueous inkjet ink may also contain a sequestering agent, such asethylenediaminetetraacetic acid (EDTA).

Inkjet Printheads

The inks according to the present invention are primarily for use inconnection with thermal inkjet printheads, although they may be used inother types of printhead, especially those where an actuator contactsthe ink. For the sake of completeness, there now follows a briefdescription of one of the Applicant's thermal inkjet printheads, asdescribed in U.S. Pat. No. 7,303,930, the contents of which is hereinincorporated by reference.

Referring to FIG. 1, there is shown part of a printhead comprising aplurality of nozzle assemblies. FIGS. 2 and 3 show one of these nozzleassemblies in side-section and cutaway perspective views.

Each nozzle assembly comprises a nozzle chamber 24 formed by MEMSfabrication techniques on a silicon wafer substrate 2. The nozzlechamber 24 is defined by a roof 21 and sidewalls 22 which extend fromthe roof 21 to the silicon substrate 2. As shown in FIG. 1, each roof isdefined by part of a nozzle plate 56, which spans across an ejectionface of the printhead. The nozzle plate 56 and sidewalls 22 are formedof the same material, which is deposited by PECVD over a sacrificialscaffold of photoresist during MEMS fabrication. Typically, the nozzleplate 56 and sidewalls 21 are formed of a ceramic material, such assilicon dioxide or silicon nitride. These hard materials have excellentproperties for printhead robustness, and their inherently hydrophilicnature is advantageous for supplying ink to the nozzle chambers 24 bycapillary action.

Returning to the details of the nozzle chamber 24, it will be seen thata nozzle opening 26 is defined in a roof of each nozzle chamber 24. Eachnozzle opening 26 is generally elliptical and has an associated nozzlerim 25. The nozzle rim 25 assists with drop directionality duringprinting as well as reducing, at least to some extent, ink flooding fromthe nozzle opening 26. The actuator for ejecting ink from the nozzlechamber 24 is a heater element 29 positioned beneath the nozzle opening26 and suspended across a pit 8. Current is supplied to the heaterelement 29 via electrodes 9 connected to drive circuitry in underlyingCMOS layers of the substrate 2. When a current is passed through theheater element 29, it rapidly superheats surrounding ink to form a gasbubble, which forces ink through the nozzle opening 26. By suspendingthe heater element 29, it is completely immersed in ink when the nozzlechamber 24 is primed. This improves printhead efficiency, because lessheat dissipates into the underlying substrate 2 and more input energy isused to generate a bubble. Typically, the heater element is comprised ofa metal or a conductive ceramic material. Examples of suitable materialsinclude titanium nitride, titanium aluminium nitride andtitanium-aluminium alloy.

As seen most clearly in FIG. 1, the nozzles are arranged in rows and anink supply channel 27 extending longitudinally along the row suppliesink to each nozzle in the row. The ink supply channel 27 delivers ink toan ink inlet passage 15 for each nozzle, which supplies ink from theside of the nozzle opening 26 via an ink conduit 23 in the nozzlechamber 24.

A MEMS fabrication process for manufacturing such printheads isdescribed in detail in U.S. Pat. No. 7,303,930, the contents of whichare herein incorporated by reference.

The operation of printheads having suspended heater elements isdescribed in detail in the Applicant's U.S. Pat. No. 7,278,717, thecontents of which are incorporated herein by reference.

The Applicant has also described thermal bubble-forming inkjetprintheads having embedded heater elements. Such printheads aredescribed in, for example, U.S. Pat. No. 7,246,876 and US 2006/0250453,the contents of which are herein incorporated by reference.

The inkjet inks of the present invention function optimally incombination with the Applicant's thermal inkjet printheads, as describedabove. However, their use is not limited to the Applicant's thermalprintheads. The inks described herein may be used in other types ofthermal bubble-forming inkjet printheads, piezoelectric printheads andthermal-bend actuated printheads (as described in, for example, U.S.Pat. No. 7,926,915; U.S. Pat. No. 7,669,967; and US 2011/0050806, thecontents of which are incorporated herein by reference) etc.

For the sake of completeness, inkjet printers incorporating theApplicant's thermal inkjet printheads are described in, for example,U.S. Pat. No. 7,201,468; U.S. Pat. No. 7,360,861; U.S. Pat. No.7,380,910; and U.S. Pat. No. 7,357,496, the contents of each of whichare herein incorporated by reference.

FIG. 4 shows a print engine 103 for a thermal inkjet printer, asdescribed in Applicant's U.S. Pat. No. 8,066,359, the contents of whichis herein incorporated by reference. The print engine 103 includes aremovable print cartridge 102, comprising a pagewidth printhead, and abank of user-replaceable ink cartridges 128. Each color channeltypically has its own ink reservoir 128 and a correspondingpressure-regulating chamber 106 for regulation of a hydrostatic pressureof ink supplied to the printhead. Hence, the print engine 103 has fiveink reservoirs 128 and five corresponding pressure-regulating chambers106. Typically, the ink channels (“color channels”) employed in thisfive-channel print engine 103 are CMYK₁K₂. The ink channel order may bearranged so as to optimize preferred ink color mixing effects at thenozzle plate of the printhead, as described in US2013/0070024, thecontents of which are herein incorporated by reference. For example, anink channel order of YK₁MK₂C may be employed where cyan (C) ispositioned furthest downstream and yellow (Y) is positioned furthestupstream.

At least one of the ink cartridges 128 may comprise an inkjet ink asdescribed herein. Although fluidic connections between the variouscomponents are not shown in FIG. 4, it will be appreciated that theseconnections are made with suitable hoses in accordance with the fluidicssystem described in, for example, U.S. Pat. No. 8,066,359, the contentsof which are incorporated herein by reference.

Experimental Section

Accelerated printhead lifetime tests were conducted in accordance withthe method described below.

Memjet® printhead integrated circuits (PHICs) having nozzle devices withsuspended uncoated resistive heater elements were mounted individuallyfor operation in a modified printing rig. The heater element materialexposed to the ink is titanium aluminium nitride.

The devices were operated to eject ink at a frequency of 11 kHz.Actuation pulse widths were controlled to replicate operation in anotherwise unmodified printer. The pulse widths are conventionallyreporting in “clocks”, which indicate the energy above a threshold pulsewidth required to eject a droplet of ink. Each clock represents Ins andthe threshold pulse width of the device is typically 47 clocks (329 ns).In the accelerated printhead lifetime tests, the devices were run at twodifferent ejection energies: “+8 clocks” [329 ns+(8×7 ns)=385 ns] and“+10 clocks” [329 ns+(10×7 ns)=399 ns].

Test patterns were printed periodically and visually inspected todetermine the health of the devices. Once the print quality had fallenbelow a predetermined threshold, the PHIC was deemed to have reached theend of its lifetime and the test was stopped. The number of ejections atthe time of printhead failure was recorded to indicate printheadlifetime.

Inks were formulated as described in Table 1 and filtered (0.2 microns)prior to use.

TABLE 1 Ink formulations for accelerated printhead lifetime tests Ink 1Ink 2 Ink 3 Ink 4 Ink 5 Ink 6 Ethylene glycol 17 17 Sulfolane 24 24 21.521.5 Diethylene glycol Triethylene glycol 3.8 3.8 N-methylpyrrolidone8.5 8.5 Propylene-1,2-glycol 8.5 8.5 Glycerol 6.2 6.2 3.5 3.5 Food Black2 Na salt 6 6 5.5 Formula (II) dye 6 6 5.5 MOPS¹ 0.2 0.2 0.2 0.2 0.2 0.2Surfynol ® 465² 0.2 0.2 0.2 0.2 0.2 0.2 Proxel ® GXL 0.2 0.2 0.2 0.2 0.20.2 Water 59.4 59.4 59.4 59.4 68.9 68.9 ¹MOPS is3-(N-morpholino)propanesulfonic acid ²Surfynol ® 465 is ethoxylated2,4,7,9-tetramethyl-5-decyne-4,7-diol

Inks 1-6 were tested in the modified printing rig described above andthe number of ejections before printhead failure, as judged by visualinspection of a standardized print quality test pattern, was determined.The results from these accelerated printhead lifetime tests are shown inTable 2.

TABLE 2 Accelerated printhead lifetime test results Millions ofejections before failure Tested Ink +8 clocks +10 clocks Ink 1 130 80Ink 2 20 10 Ink 3 170 130 Ink 4 55 40 Ink 5 130 80 Ink 6 60 30

From Table 2, the following observations can be made:

(1) Inks 1, 3 and 5 containing the black dye of formula (II) hadsignificantly longer printhead lifetimes than Inks 2, 4 and 6 containingFood Black 2. These longer printhead lifetimes were consistentlyobserved irrespective of the formulation and despite the fact that alldyes tested had sodium counterions.(2) Of the inks containing the black dye of formula (II), Ink 3formulated with sulfolane and triethylene glycol co-solvents wasgenerally superior to Ink 1 and Ink 5.(3) Of the inks containing Food Black 2, Inks 4 and 6 had comparableprinthead lifetimes, while Ink 2 had a very poor printhead lifetime.(4) Ink 3 containing the black dye of formula (II) in a formulationcomprising sulfolane and triethylene glycol co-solvents significantlyoutperformed all other inks.

From these results, it was concluded that there was synergism betweenthe dye of formula (II), sulfolane and triethylene glycol, whichsignificantly improved the lifetime of Memjet® printheads.

It will, of course, be appreciated that the present invention has beendescribed by way of example only and that modifications of detail may bemade within the scope of the invention, which is defined in theaccompanying claims.

1. An inkjet ink comprising: (i) a disazo dye of formula (I):

wherein: A is a C₆₋₁₄ aryl group having 0, 1, 2, 3, 4 or 5 substituentsselected from the group consisting of: —SO₃M, nitro, carboxyl, halogen,hydroxyl, amino, C₁₋₄ alkylamino, C₁₋₄ alkoxy, C₁₋₄ alkyl, C₁₋₄haloalkyl, cyano, sulfonamide, carbamoyl, C₁₋₄ alkylamido and C₁₋₄alkoxycarbonyl; B is a C₆₋₁₄ aryl group having 0, 1, 2, 3, 4 or 5substituents selected from the group consisting of: —SO₃M, nitro,carboxyl, halogen, hydroxyl, amino, C₁₋₄ alkylamino, C₁₋₄ alkoxy, C₁₋₄alkyl, C₁₋₄ haloalkyl, cyano, sulfonamide, carbamoyl, C₁₋₄ alkylamidoand C₁₋₄ alkoxycarbonyl; each M is independently selected from the groupconsisting of: hydrogen, lithium, sodium, potassium, ammonium andquaternary ammonium; and the dye of formula (I) comprises at least 3groups of formula —SO₃M; (ii) a glycol compound selected from the groupconsisting of: triethylene glycol, tetraethylene glycol andpentaethylene glycol; and (iii) water.
 2. The inkjet ink of claim 1,wherein the glycol compound is present in an amount ranging from 1 to 20wt. %.
 3. The inkjet ink of claim 1, further comprising glycerol.
 4. Theinkjet ink of claim 1, further comprising a non-ionic surfactant.
 5. Theinkjet ink of claim 1, wherein M is sodium.
 6. The inkjet ink of claim1, wherein A is a phenyl or naphthyl group having 1, 2 or 3 substituentsselected from the group consisting of: —SO₃M, nitro, carboxyl, halogen,hydroxyl, amino, C₁₋₄alkoxy and C₁₋₄ alkyl; and B is a phenyl ornaphthyl group having 1, 2 or 3 substituents selected from the groupconsisting of: —SO₃M, nitro, carboxyl, halogen, hydroxyl, amino, C₁₋₄alkoxy and C₁₋₄ alkyl.
 7. A method of improving a lifetime of an inkjetprinthead, the method comprising the steps of: supplying an inkaccording to claim 1 to nozzle chambers of the printhead, each nozzlechamber having an associated actuator in contact with the ink; andactuating one or more of the actuators to eject ink from the printhead.8. The method of claim 7, wherein each actuator comprises a resistiveheater element.
 9. The method of claim 8, wherein the heater element isuncoated.
 10. The method of claim 9, wherein the heater element iscomprised of a material selected from the group consisting of: atitanium alloy; titanium nitride; and a nitride of a titanium alloy. 11.The method of claim 10, wherein the printhead has a lifetime of at least100 million ejections.